Question Engine starts in space

[IronRain]

I have a question about engine starts once a rocket is out of the atmosphere.
Some launchers have a lox/lh2 pre-start. Why is this?
I've once read somewhere that it cools some engine components down before ignition. Is this true?

 

[Urwumpe]

I have a question about engine starts once a rocket is out of the atmosphere.
Some launchers have a lox/lh2 pre-start. Why is this?
I've once read somewhere that it cools some engine components down before ignition. Is this true?

Yes, you need a prestart sequence for preventing that valves, pumps or turbine shafts deform and get damaged when contact with the cryogen fuel cools them quickly and makes the parts contract.

Thus it is also popular for cryogen rocket engines to avoid restarts.

Also, you often need to provide helium pressure for seals and bearings, that has to be thermally conditioned with the engine parts, because otherwise, the cold fuel would also cool the helium gas down and create a mild vacuum in places where you badly need a light overpressure.

It is less a problem for kerosene/LOX engines, since only one component of the fuel is cryogen.

 

[IronRain]

Thank you, it's clear for me now! :thumbup: